The invention relates to a method of operating a spark ignition engine with direct fuel injection in which the engine is operated selectively with charge stratification or with a homogeneous charge.
During operation with charge stratification, referred to as stratified-charge operation, the fuel is injected during the compression stroke, preferably with a significant excess of air, in such a way that the fuel is in the combustion chamber in stratified form whereby the fuel/air mixture is reliably ignited despite the excess of air. Operation with charge stratification allows lower fuel consumption in comparison with conventional external mixture formation, mainly because the charge change, pumping and wall-heat losses are lower. However, stratified-charge operation is restricted to the part-load engine operating range, whereas, at a higher engine loads, the direct-injection spark ignition engine is operated homogeneously, i.e. the combustion chamber is supplied with the fuel in a homogeneously distributed manner, the fuel injection taking place during the intake stroke with a stoichiometric fuel/air ratio.
DE-A1 39 40 752 discloses a method for operating a spark ignition engine without a throttle valve, in which the opening times of the inlet valves, via which a fuel/air mixture prepared in the intake pipe is introduced into the respective combustion chamber, and the fuel quantities are set in a variable manner as a function of the engine speed and the position of the accelerator pedal. This is intended to ensure that the masses of air and fuel are always optimally matched to one another under all driving conditions, even in the case of sudden changes in the position of the accelerator pedal. In this way, a high level of refinement without misfiring is obtained while low fuel consumption and pollutant emissions are generated. When required, this variable control of the inlet-valve opening times is also used to achieve internal exhaust-gas recirculation by controlling the respective inlet valve in such a way that it is opened while the associated cylinder piston is still expelling exhaust gas from the respective combustion chamber. Provisions can also be made to provide two inlet ducts for each combustion chamber, namely a swirl duct and a filling duct, each of which has its own controllable inlet valve. The air entering via the swirl duct is subject to a high degree of swirl, leading to good mixture preparation, while the air flowing through the filling duct enters the combustion chamber with as little resistance as possible and hence a low degree of swirl. In the lower load range, the filing-duct inlet valves are then held continuously closed. In a higher load range, the maximum possible quantity of air is made to enter the cylinder via the swirl-duct inlet valves. Depending on the air-quantity requirements, individual cylinders may remain completely switched off, or the remaining air mass is drawn in via the filling-duct inlet valves.
German Patent Application 197 12 356.2 disclose a method for reducing noxious exhaust emissions from a direct-injection spark ignition engine operated on a lean fuel/air mixture, in which there is alternation between stratified-charge operation ad homogeneous charge operation for the purpose of regenerating an NOx adsorber-type catalytic converter. In the part-load range, the engine is normally operated with a lean mixture in stratified-charge mode. As soon as regeneration of the adsorber-type catalytic converter is necessary, during which process the nitrogen oxides adsorbed by the converter are released and reduced, a switch is made to homogeneous charge engine operation such that, in a first step, the air quantity in the combustion chamber is reduced by means of changes in timing of the valve or by means of throttling devices in an associated intake duct. In a second step, a transition is made from stratified-charge operation to homogeneous charge operation with a rich fuel/air mixture. The air quantity can be reduced, in particular, by delayed closure of the inlet valves after bottom dead center or by their early closure after the top dead center of the combustion-chamber piston.
It is the object of the invention to provide an optimized method for operating a direct-injection spark ignition engine.
In a method for operating a spark ignition engine with direct fuel injection, in which the engine is operated either with charge stratification or with a homogeneous charge, the open and/or closed positions of the inlet and exhaust valves are controlled with variable timing depending on the engine operating condition. The timings are determined on the basis of basic timing settings, which are provided in basic timing performance graphs for operation with charge stratification and for homogeneous charge operation. The basic timing settings are modified by means of correction values to raise the temperature of the exhaust gas for internal exhaust-gas recirculation and for initiating a movement of the charge in the cylinders.
This method involves variable valve timing, during which the inlet and exhaust valves are moved in a controllable manner to their open and closed positions depending on engine operating conditions. In this method, the determination of the valve timings is based on basic timings, which are provided in a controllable manner by respective basic timing performance graphs for operation with charge stratification, on one hand, and for homogeneous operation, on the other hand. By means of this variable valve timing, the engine can be operated either with charge stratification or with a homogeneous charge. In addition, the basic timing values specified by these two performance graphs can be modified by means of timing correction values in order to provide for an increase in the temperature of the exhaust gas or to provide for internal exhaust-gas recirculation and/or a movement of the charge as part of the variable valve timing. Increasing the temperature of the exhaust gas is appropriate particularly where there is a downstream catalytic converter since those generally have a satisfactory conversion performance only at a certain elevated temperature level of the exhaust gas, as is the case, for example, with NOx adsorber-type catalytic converters. Special measure of raising the temperature of the exhaust gas, may be provided, whereby the exhaust-gas temperature range can be adjusted to be favorable for the catalytic converter to prevent its temperature from falling too low during low engine load and stratified-charge operation.
Two particularly advantageous measures for raising the temperature of the exhaust gas by an appropriate selection of timing corrections are provided.
This makes it possible to raise the temperature of the exhaust gas by reducing the air quantity without significant throttling losses and without a significant rise in fuel consumption.
Internal exhaust-gas recirculation is made possible by early opening of the inlet valves while the respective operating cycle is still in the exhaust-gas expulsion phase.
If the spark ignition engine has at least two inlet valves for any particular combustion chamber a movement of the charge by appropriate differences in the control of the two inlet valves can be initiated.
Advantageous embodiments of the invention will be illustrated below on the basis of the drawings: